Water treatment method for heavy oil production using calcium sulfate seed slurry evaporation

ABSTRACT

A process for treating produced water to generate high pressure steam. Produced water from heavy oil recovery operations is treated by first removing oil and grease. Feedwater is then acidified and steam stripped to remove alkalinity and dissolved non-condensable gases. Pretreated produced water is then fed to an evaporator. Up to 95% or more of the pretreated produced water stream is evaporated to produce (1) a distillate having a trace amount of residual solutes therein, and (2) evaporator blowdown containing substantially all solutes from the produced water feed. The distillate may be directly used, or polished to remove the trace residual solutes before being fed to a steam generator. Steam generation in a packaged boiler, such as a water tube boiler having a steam drum and a mud drum with water cooled combustion chamber walls, produces 100 % quality high pressure steam for down-hole use.

RELATED PATENT APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.11,149,072, filed Jun. 8, 2005, issued on Oct. 21, 2008 as U.S. Pat. No.7,438,129, which was a Continuation-In-Part of prior U.S. patentapplication Ser. No. 10/868,745, filed Jun. 9, 2004, now U.S. Pat. No.7,150,320B2 issued Dec. 19, 2006, entitled WATER TREATMENT METHOD FORHEAVY OIL PRODUCTION, which was a Continuation-In-Part of prior U.S.patent application Ser. No. 10/307,250, filed Nov. 30, 2002, now U.S.Pat. No. 7,077,201B2 issued Jul. 18, 2006, entitled WATER TREATMENTMETHOD FOR HEAVY OIL PRODUCTION, which was a Continuation-In-Part ofprior U.S. patent application Ser. No. 09/566,622, filed May 8, 2000,now U.S. Pat. No. 6,733,636B1 issued May 11, 2004, entitled WATERTREATMENT METHOD FOR HEAVY OIL PRODUCTION, which claimed priority fromprior U.S. Provisional Patent Application Ser. No. 60/133,172, filed onMay 7, 1999. Also, U.S. patent application Ser. No. 11/149,072 claimedpriority from U.S. Provisional Patent Application Ser. No. 60/578,810,filed Jun. 9, 2004. The disclosures of each of the above identifiedpatents or patent applications are incorporated herein in their entiretyby this reference, including the specification, drawing, and claims ofeach patent or application.

COPYRIGHT RIGHTS IN THE DRAWING

A portion of the disclosure of this patent document contains materialthat is subject to copyright protection. The applicant no objection tothe facsimile reproduction by anyone of the patent document or thepatent disclosure, as it appears in the Patent and Trademark Officepatent file or records, but otherwise reserves all copyright rightswhatsoever.

TECHNICAL FIELD

The invention disclosed and claimed herein relates to treatment of waterto be used for steam generation in operations which utilize steam torecover oil from geological formations. More specifically, thisinvention relates to novel, improved techniques for efficiently andreliably generating from oil field produced waters, in high pressuresteam generators, the necessary steam for down-hole use in heavy oilrecovery operations.

BACKGROUND

Steam generation is necessary in heavy oil recovery operations. This isbecause in order to recover heavy oil from certain geologic formations,steam is required to increase the mobility of the sought after oilwithin the formation. In prior art systems, oil producers have oftenutilized once-through type steam generators (“OTSG's). As generallyutilized in the industry, once through steam generators—OTSG's—usuallyhave high blowdown rates, often in the range of from about 20% to about30% or thereabouts. Such a blowdown rate leads to significant thermaland chemical treatment inefficiencies. Also, once through steamgenerators are most commonly provided in a configuration and withprocess parameters so that steam is generated from a feedwater in asingle-pass operation through boiler tubes that are heated by gas or oilburners. Typically, such once through steam generators operate at fromabout 1000 pounds per square inch gauge (psig) to about 1600 psig or so.In some cases, once through steam generators are operated at up to asmuch as about 1800 psig. Such OTSG's often operate with a feedwater thathas from about 2000 mg/L to about 8000 mg/L of total dissolved solids.As noted in FIG. 1, which depicts the process flow sheet of a typicalprior art water treatment system 10, such a once through steam generator12 provides a low quality or wet steam, wherein about eighty percent(80%) quality steam is produced. In other words, the 80% quality steam14 is about 80% vapor, and about 20% liquid, by weight percent. Thesteam portion, or high pressure steam produced in the steam generatorsis injected via steam injection wells 16 to fluidize as indicated byreference arrows 18, along or in combination with other injectants, theheavy oil formation 20, such as oils in tar sands formations. Theinjected steam 14 eventually condenses and an oil/water mixture 22results, and which mixture migrates through the formation 20 asindicated by reference arrows 24. The oil/water mixture 22 is gatheredas indicated by reference arrows 26 by oil/water gathering wells 30,through which the oil/water mixture is pumped to the surface. Then, thesought-after oil is sent to an oil/water separator 32 in which the oilproduct 34 separated from the water 35 and recovered for sale. Theproduced water stream 36, after separation from the oil, is furtherde-oiled in a de-oiling process step 40, normally by addition of ade-oiling polymer 42 or by other appropriate processes. Such a de-oilingprocess usually results in generation of an undesirable waste oil/solidssludge 44. However, the de-oiled produced water stream 46 is thenfurther treated for reuse.

The design and operation of the water treatment plant which treats thede-oiled produced water stream 46, i.e., downstream of the de-oilingunit 40 and upstream of injection well 16 inlet 48, is the key to theimprovement(s) described herein.

Most commonly in prior art plants such as plant 10, the water is sent tothe “once-through” steam generators 12 for creation of more steam 14 foroil recovery operations. The treated produced water stream 12F which isthe feed stream for the once through steam generator, at time of feed tothe steam generator 12, is typically required to have less than about8000 parts per million (“PPM”) of total dissolved solids (“TDS”). Lessfrequently, the treated produced water stream 12F may have up to about12000 parts per million (as CaCO3 equivalent) of total dissolved solids,as noted in FIG. 8. Further, it is often necessary to meet otherspecific water treatment parameters before the water can be reused insuch once-through steam generators 12 for the generation of highpressure steam.

In most prior art water treatment schemes, the de-oiled recovered water46 must be treated in a costly water treatment plant sub-system 10 ₁before it can be sent to the steam generators 12. Treatment of waterbefore feed to the once-through steam generators 12 is often initiallyaccomplished by using a warm lime softener 50, which removes hardness,and which also removes some silica from the de-oiled produced waterfeedstream 46. Various softening chemicals 52 are usually necessary,such as lime, flocculating polymer, and perhaps soda ash. The softenerclarifier 54 underflow 56 produces a waste sludge 58 which must befurther handled and disposed. Then, an “after-filter” 60 is oftenutilized on the clarate stream 59 from the softener clarifier 54, toprevent carry-over from the softener clarifier 54 of any precipitate orother suspended solids, which substances are thus accumulated in afiltrate waste stream 62. For polishing, an ion exchange step 64,normally including a hardness removal step such as a weak acid cation(WAC) ion-exchange system that can be utilized to simultaneously removehardness and the alkalinity associated with the hardness, is utilized.The ion exchange systems 64 require regeneration chemicals 66 as is wellunderstood by those of ordinary skill in the art and to which thisdisclosure is directed. As an example, however, a WAC ion exchangesystem is usually regenerated with hydrochloric acid and caustic,resulting in the creation of a regeneration waste stream 68. Overall,such prior art water treatment plants are relatively simple, but, resultin a multitude of liquid waste streams or solid waste sludges that mustbe further handled, with significant additional expense.

In one relatively new heavy oil recovery process, known as the steamassisted gravity drainage heavy oil recovery process (the “SAGD”process), it is preferred that one hundred percent (100%) quality steambe provided for injection into wells (i.e., no liquid water is to beprovided with the steam to be injected into the formation). Such atypical prior art system 11 is depicted in FIG. 2. However, givenconventional prior art water treatment techniques as just discussed inconnection with FIG. 1, the 100% steam quality requirement presents aproblem for the use of once through steam generators 12 in such aprocess. That is because in order to produce 100% quality steam 70 usinga once-through type steam generator 12, a vapor-liquid separator 72 isrequired to separate the liquid water from the steam. Then, the liquidblowdown 73 recovered from the separator is typically flashed severaltimes in a series of flash tanks F₁, F₂, etc. through F_(N) (where N isa positive integer equal to the number of flash tanks) to successivelyrecover as series of lower pressure steam flows S₁, S₂, etc. which maysometimes be utilized for other plant heating purposes. After the lastflashing stage F_(N), a residual hot water final blowdown stream 74 mustthen be handled, by recycle and/or disposal. The 100% quality steam isthen sent down the injection well 16 and injected into the desiredformation 20. Fundamentally, though, conventional treatment processesfor produced water used to generate steam in a once-through steamgenerator produces a boiler blowdown which is roughly twenty percent(20%) of the feedwater volume. This results in a waste brine stream thatis about fivefold the concentration of the steam generator feedwater.Such waste brine stream must be disposed of by deep well injection, orif there is limited or no deep well capacity, by further concentratingthe waste brine in a crystallizer or similar system which produces a drysolid for disposal.

As depicted in FIG. 3, another method which has been proposed forgenerating the required 100% quality steam for use in the steam assistedgravity drainage process involves the use of boilers 80, which may bepackaged, factory built boilers of various types or field assembledboilers with mud and steam drums and water wall piping. Various methodscan be used for producing water of a sufficient quality to be utilizedas feedwater 80F to a boiler 80. One method which has been developed foruse in heavy oil recovery operations involves de-oiling 40 of theproduced water 36, followed by a series of physical-chemical treatmentsteps. Such treatment steps normally include a series of unit operationsas warm lime softening 54, followed by filtration 60 for removal ofresidual particulates, then an organic trap 84 (normally non-ionic ionexchange resin) for removal of residual organics. The organic trap 84may require a regenerant chemical supply 85, and, in any case, producesa waste 86, such as a regenerant waste. Then, a pre-coat filter 88 canbe used, which has a precoat filtrate waste 89. In one alternateembodiment, an ultrafiltration (“UF”) unit 90 can be utilized, whichunit produces a reject waste stream 91. Then, effluent from the UF unit90 or precoat filter 88 can be sent to a reverse osmosis (“RO”) system92, which in addition to the desired permeate 94, produces a rejectliquid stream 96 that must be appropriately handled. Permeate 94 fromthe RO system 92, can be sent to an ion exchange unit 100, typically butnot necessarily a mixed bed demineralization unit, which of courserequires regeneration chemicals 102 and which consequently produces aregeneration waste 104. And finally, the boiler 80 produces a blowdown110 which must be accommodated for reuse or disposal.

The prior art process designs, such as depicted in FIG. 3, for utilizingpackaged boilers in heavy oil recovery operations, have a high initialcapital cost. Also, such a series of unit process steps involvessignificant ongoing chemical costs. Moreover, there are many wastestreams to discharge, involving a high and ongoing sludge disposal cost.Further, where membrane systems such as ultrafiltration 90 or reverseosmosis 92 are utilized, relatively frequent replacement of membranes106 or 108, respectively, may be expected, with accompanying on-goingperiodic replacement costs. Also, such a process scheme can be laborintensive to operate and to maintain.

In summary, the currently known and utilized methods for treating heavyoil field produced waters in order to generate high quality steam fordown-hole use are not entirely satisfactory because:

-   -   such physical-chemical treatment process schemes are usually        quite extensive, are relatively difficult to maintain, and        require significant operator attention;    -   such physical-chemical treatment processes require many chemical        additives which must be obtained at considerable expense, and        many of which require special attention for safe handling;    -   such physical-chemical treatment processes produce substantial        quantities of undesirable sludges and other waste streams, the        disposal of which is increasingly difficult, due to stringent        environmental and regulatory requirements.

It is clear that the development of a simpler, more cost effectiveapproach to produced water treatment would be desirable in the processof producing steam in heavy oil production operations. Thus, it can beappreciated that it would be advantageous to provide a new producedwater treatment process which minimizes the production of undesirablewaste streams, while minimizing the overall costs of owning andoperating a heavy oil recovery plant.

SOME OBJECTS, ADVANTAGES, AND NOVEL FEATURES

The new water treatment process(es) disclosed herein, and variousembodiments thereof, can be applied to heavy oil production operations.Such embodiments are particularly advantageous in that they minimize thegeneration of waste products, and are otherwise superior to watertreatment processes heretofore used or proposed in the recovery ofbitumen from tar sands or other heavy oil recovery operations.

From the foregoing, it will be apparent to the reader that one of theimportant and primary objectives resides in the provision of a novelprocess, including several variations thereof, for the treatment ofproduced waters, so that such waters can be re-used in producing steamfor use in heavy oil recovery operations.

Another important objective is to simplify process plant flow sheets,i.e., minimize the number of unit processes required in a watertreatment train, which importantly simplifies operations and improvesquality control in the manufacture of high purity water for down-holeapplications.

Other important but more specific objectives reside in the provision ofvarious embodiments for an improved water treatment process forproduction of high purity water for down-hole use in heavy oil recovery,which embodiments may:

-   -   in one embodiment, eliminate the requirement for flash        separation of the high pressure steam to be utilized downhole        from residual hot pressurized liquids;    -   eliminate the generation of softener sludges;    -   minimize the production of undesirable liquid or solid waste        streams;    -   minimize operation and maintenance labor requirements;    -   minimize maintenance materiel requirements;    -   minimize chemical additives and associated handling        requirements;    -   increase reliability of the OTSG's, when used in the process;    -   decouple the de-oiling operations from steam production        operations; and    -   reduce the initial capital cost of water treatment equipment.

Other important objectives, features, and additional advantages of thevarious embodiments of the novel process disclosed herein will becomeapparent to the reader from the foregoing and from the appended claimsand the ensuing detailed description, as the discussion below proceedsin conjunction with examination of the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

In order to enable the reader to attain a more complete appreciation ofthe novel water treatment process disclosed and claimed herein, and thevarious embodiments thereof, and of the novel features and theadvantages thereof over prior art processes, attention is directed tothe following detailed description when considered in connection withthe accompanying figures of the drawing, wherein:

FIG. 1 shows one typical prior art process, namely a generalized processflow diagram for a physical-chemical water treatment process configuredfor use in heavy oil recovery operations.

FIG. 2 shows another prior art process, namely a generalized processflow diagram for a physical-chemical water treatment process as used ina steam assisted gravity drainage (SAGD) type heavy oil operation.

FIG. 3 shows yet another prior art physical-chemical treatment processscheme, also as it might be applied for use in steam assisted gravitydrainage (SAGD) type heavy oil recovery operations.

FIG. 4 shows one embodiment of an evaporation based water treatmentprocess, illustrating the use of a seeded slurry evaporation basedprocess in combination with the use of packaged boilers for steamproduction, as applied to heavy oil recovery operations.

FIG. 5 shows another embodiment for an evaporation based water treatmentprocess for heavy oil production, illustrating the use of a seededslurry evaporation process in combination with the use of once-throughsteam generators for steam production, as applied to heavy oil recoveryoperations, which process is characterized by feed of evaporatordistillate to once-through steam generators without the necessity offurther pretreatment.

FIG. 6 shows a common variation for the orientation of injection andgathering wells as utilized in heavy oil recovery, specifically showingthe use of horizontal steam injection wells and of horizontal oil/watergathering wells, as often employed in a steam assisted gravity drainageheavy oil gathering project.

FIG. 7 shows the typical feedwater quality requirements for steamgenerators which produce steam in the 1000 pounds per square inch gaugerange, or thereabouts, for conventional steam boiler installations.

FIG. 8 shows the typical feedwater quality requirements for steamgenerators which produce steam in the 1000 pounds per square inch gaugerange, or thereabouts, for once-through type steam generatorinstallations.

FIG. 9 provides a simplified view of a vertical tube falling filmevaporator operating in a seeded slurry mode in the treatment ofproduced water from heavy oil operations, for production of distillatefor reuse in once through steam generators or in conventional steamboilers.

FIG. 10 shows further details of the use of evaporators operating in aseeded slurry mode, illustrated by use of falling film evaporators, andindicates selected injection points for acidification of the feedwaterand for control of pH in the evaporator via optional injection of aselected base such as sodium hydroxide.

FIG. 11 illustrates the solubility of silica in water as a function ofpH at 25° C. when such silica species are in equilibrium with amorphoussilica, as well as the nature of such soluble silica species (moleculeor ion) at various concentration and pH ranges.

FIG. 12 diagrammatically illustrates functional internal details of theoperation of a falling film evaporator operating in a seeded slurrymode, which evaporator type would be useful in the evaporation ofproduced waters from heavy oil production; details illustrated includethe production of steam from a falling brine film, by a heat exchangerelationship from condensation of steam on a heat exchange tube, and thedownward flow of such steam condensate (distillate) by gravity for thecollection of such condensate (distillate) above the bottom tube sheetof the evaporator.

The foregoing figures, being merely exemplary, contain various elementsthat may be present or omitted from actual process implementationsdepending upon the circumstances. An attempt has been made to draw thefigures in a way that illustrates at least those elements that aresignificant for an understanding of the various embodiments and aspectsof the invention. However, various other elements of the unique processmethods, and the combination of apparatus for carrying out the methods,are also shown and briefly described to enable the reader to understandhow various features, including optional or alternate features, may beutilized in order to provide an efficient, low cost process design whichcan be implemented in a desired throughput size and physicalconfiguration for providing optimum water treatment plant design andoperation.

DESCRIPTION

Many steam assisted heavy oil recovery schemes, such as a steam assistedgravity drainage (SAGD) heavy oil recovery process injection andrecovery well arrangements of the type depicted in FIG. 6, mostefficiently utilize a 100% quality steam supply 70. It would thereforebe desirable to produce such a steam supply by an efficient processscheme such as I have found may be provided by evaporation based heavyoil produced water treatment method(s). Various embodiments and detailsof such evaporation based produced water treatment method(s) aredepicted in FIGS. 4, 5, 6, 9, 10 and 12.

As depicted in FIG. 6, in a SAGD process, horizontal injection wells 16′and horizontal oil/water gathering wells 30′ are advantageously utilizedspaced apart within an oil bearing formation 20. As particularlyillustrated in FIGS. 4 and 5, a process for the use of an evaporationbased water treatment system 120 has been developed to treat producedwater, in order to produce high quality steam for use in further heavyoil recovery. Conceptually, such an evaporative water treatment processmay, in one embodiment, be situated process wise—that is, water flowwise—between the point of receipt of a de-oiled produced water stream 46and the point of steam injection at well head 48 of injection well 16.The process, in combination with the steam injection well 16, oilrecovery well 30, and related oil water separation equipment 32 andde-oiling equipment 40, and boilers 80 as shown in FIG. 4, oralternately, once through steam generators 12 as shown in FIG. 5, cansubstantially reduce capital costs and can minimize ongoing operationand maintenance costs of heavy oil recovery installations. Boilers 80may be packaged, factory built boilers of various types or fieldassembled boilers with mud and steam drums and water wall piping, ormore generally, conventional steam boilers. In some locales, such asnorthern Canada, the possibility of elimination of the need for handlingof waste sludges and other waste streams made possible by theevaporation based water treatment system 120 may be especiallyimportant, since it may be difficult to work with such waste materialsduring the extremely cold winter months.

It has been observed that it may be desirable in some instances to use apackaged boiler 80 to produce the required steam 70, rather than toutilize a traditional once-through type steam generator 12 to produce80% quality steam 14 and then utilize separator(s) 130 to separate steam132 from liquid 134. It is noteworthy in such an economic processevaluation that packaged boilers 80 are often less expensive on acapital cost basis and on an operating cost basis than once-through typeoil-field steam generators 12. Also, package boilers can be utilized toproduce pure steam 70, and thus produce only a minimal liquid blowdownstream 110. Also, as shown in FIGS. 4 and 5, boiler blowdown stream canbe either sent to the evaporator feed tank 210, or injected into thesump reservoir 152 of evaporator 140, such as via line 111, or into arecirculating brine via line 111′. One type of packaged boiler suitablefor use in the process described herein is a water tube boiler having alower mud drum and an upper steam drum and water cooled sidewallssubstantially extending therebetween in a manner which encloses acombustion chamber. However, most such packaged boilers require a muchhigher quality feed water 80F than is the case with requirements forfeedwater 12F for a once-through type steam generator. As a result, inone embodiment, the process disclosed herein includes an evaporationunit 140 based approach to packaged boiler 80 feedwater 80Fpretreatment. In other words, the de-oiled produced water 46 generatedcan be advantageously treated by an evaporative process operating in aseeded slurry mode, particularly if the oil in the de-oiled producedwater is reduced reliably to a selected low level of less than about 20parts per million, or more preferably to less than about 10 parts permillion, and provides a significantly improved method for produced watertreatment in heavy oil production.

An oil/water mixture 22 is pumped up through oil gathering wells 30. Theoil water mixture 22 is sent to a series of oil/water separators 32. Anoil product 34 is gathered for further conditioning, transport, andsale. The produced water 36 which has been separated from the oil/watermixture 22 is then sent to a produced water de-oiling step 40, which maybe accomplished in dissolved air flotation units with the assistance ofthe addition of a de-oiling polymer 42, or by other appropriate unitprocesses, to achieve a preselected low residual oil level such as lessthan 20 parts per million.

In the water treatment method disclosed herein, the de-oiled producedwater 46 is treated and conditioned for feed to one or more mechanicalvapor recompression evaporator units 140 (normally, multiple redundantunits) to concentrate the incoming produced water stream 46. Thenecessary treatment and conditioning prior to the evaporator unit 140can be efficiently accomplished, but may vary somewhat based onfeedwater chemistry—i.e. the identity and distribution of variousdissolved and suspended solids—and on the degree of concentrationselected for accomplishment in evaporator units 140.

In one embodiment, it may be necessary or appropriate to add acid byline 144, or at an appropriate point upstream of the feed tank 210 whendesired such as via line 146′. A suitable acid may be sulfuric acid orhydrochloric acid, which is effective to lower the pH sufficiently sothat bound carbonates are converted to free gaseous carbon dioxide,which is removed, along with other non-condensable gases 148 dissolvedin the feedwater 46 such as oxygen and nitrogen, in an evaporatorfeedwater deaerator 150. However, use of acid 144 is this manner isoptional, and can sometimes be avoided if feedwater chemistry and theconcentration limits of scale forming species are sufficiently low atthe anticipated concentration factor utilized in evaporator 140. For pHcontrol, as seen in FIG. 10, it may be useful to add a selected basesuch as caustic 232 to the concentrated brine recirculating in theevaporator 140, which can be accomplished by direct injection of aselected base such as caustic 232 into the sump 141, as indicated byline 157, or by feed of a selected base such as caustic 232 into thesuction of recirculation pump 153, as indicated by line 159. However, ifthe produced water contains an appreciable amount of calcium andsulfate, the mechanical vapor recompression evaporator 140 may in oneembodiment be operated using a calcium sulfate seeded-slurry technique,normally in a near neutral pH range. That mode of operation can be madepossible by the substantial elimination of carbonate alkalinity beforethe feedwater is introduced into the evaporator 140. Then, theevaporator 140 may be operated a seeded-slurry mode wherein calciumsulfate and silica co precipitated recirculating seed crystals, whichavoids scaling of the heat transfer surfaces.

At feedwater heat exchanger, the feedwater pump 149 is used to providesufficient pressure to send feedwater from the evaporator feed tank 210through the feedwater heat exchanger 148, prior to the deaerator 150. Inthe opposite direction, the distillate pump 143 moves distillate 180through the feedwater heat exchanger 148, so that the hot distillate isused to heat the feedwater stream directed toward the deaerator 150.

The conditioned feedwater 151 is sent as feedwater to evaporator 140.The conditioned feedwater 151 may be directed to the inlet ofrecirculation pump 153, or alternately, directed to the sump 141 ofevaporator 140 as indicated by broken line 151′ in FIG. 10. Concentratedbrine 152 in the evaporator 140 is recirculated via pump 153, so only asmall portion of the recirculating concentrated brine is removed on anyone pass through the evaporator 140. In the evaporator 140, the solutesin the feedwater 46 are concentrated via removal of water from thefeedwater 46. As depicted in FIGS. 10 and 12, an evaporator 140 is inone embodiment provided in a falling film configuration wherein a thinbrine film 154 is provided by distributors 155 and then falls inside ofa heat transfer element, e.g. tube 156. A small portion of the water inthe thin brine film 154 is extracted in the form of steam 160, via heatgiven up from heated, compressed steam 162 which is condensing on theoutside of heat transfer tubes 156. Thus, the water is removed in theform of steam 160, and that steam is compressed through the compressor164, and the compressed steam 162 is condensed at a heat exchange tube156 in order to produce yet more steam 160 to continue the evaporationprocess. The condensing steam on the outer wall 168 of heat transfertubes 156, which those of ordinary skill in the evaporation arts and towhich this disclosure is directed may variously refer to as eithercondensate or distillate 180, is in relatively pure form, low in totaldissolved solids. In one embodiment, such distillate contains less than10 parts per million of total dissolved solids of non-volatilecomponents. Since, as depicted in the embodiments shown in FIGS. 4, 5,9, and 10, a single stage of evaporation is provided, such distillate180 may be considered to have been boiled, or distilled, once, and thuscondensed but once.

Prior to the initial startup of the evaporator 140 in the seeded-slurrymode, the evaporator, which in such mode may be provided in afalling-film, mechanical vapor recompression configuration, the fluidcontents of the unit are “seeded” by the addition of calcium sulfate(gypsum). The circulating solids within the brine slurry serve asnucleation sites for subsequent precipitation of calcium sulfate 272, aswell as silica 274. Such substances both are precipitated as an enteringfeedwater is concentrated. Importantly, the continued concentratingprocess produces additional quantities of the precipitated species, andthus creates a continuing source of new “seed” material as theseparticles are broken up by the mechanical agitation, particularly by theaction of the recirculation pump 153.

In order to avoid silica and calcium sulfate scale buildup in theevaporator 140, calcium sulfate seed crystals 272 are continuouslycirculated over the wetted surfaces, i.e., the falling film evaporatortubes 156, as well as other wetted surfaces in the evaporator 140.Through control of slurry concentration, seed characteristics, andsystem geometry, the evaporator can operate in the otherwise scaleforming environment. The thermo chemical operation within the evaporator140 with regard to the scale prevention mechanism is depicted in FIG.12. As the water is evaporated from the brine film 154 inside the tubes156, the remaining brine film becomes super saturated and calciumsulfate and silica start to precipitate. The precipitating materialpromotes crystal growth in the slurry rather than new nucleation thatwould deposit on the heat transfer surfaces; the silica crystals attachthemselves to the calcium sulfate crystals. This scale preventionmechanism, called preferential precipitation, has a proven capability topromote clean heat transfer surfaces 260. The details of oneadvantageous method for maintaining adequate seed crystals inpreferentially precipitation systems is set forth in U.S. Pat. No.4,618,429, issued Oct. 21, 1986to Howard R. Herrigel, the disclosure ofwhich is incorporated into this application in full by this reference.

It is to be understood that the falling film evaporator 140 design isprovided only for purposes of illustration and thus enabling the readerto understand the water treatment process(es) taught herein, and is notintended to limit the process to the use of such evaporator design, asthose in the art will recognize that other designs, such as, forexample, a forced circulation evaporator, or a rising film evaporator,may be alternately utilized with the accompanying benefits and/ordrawbacks as inherent in such alternative evaporator designs.

In any event, in a falling film evaporator embodiment, the distillate180 descends by gravity along tubes 156 and accumulates above bottomtube sheet 172, from where it is collected via condensate line 174. Asmall portion of steam in equilibrium with distillate 180 may be sentvia line 172 to the earlier discussed deaerator 150 for use in masstransfer, i.e, heating and steam stripping descending liquids in apacked tower to remove non-condensable gases 148 such as carbon dioxide.However, the bulk of the distillate 180 is removed as a liquid via line180′, and may optionally be sent for further treatment in a distillatetreatment plant, for example such as depicted in detail in FIG. 4, or asmerely depicted in functional form as feed 181 _(F) for plant 181 inFIG. 5, to ultimately produce a product water 181 _(P) which is suitablefor evaporator feedwater, such as feedwater 80F′ in the case wherepackaged boilers 80 are utilized as depicted in FIG. 4. The plant 181also normally produces a reject stream 181 _(R) which may be recycled tothe evaporator feed tank 210 or other suitable location for reprocessingor reuse. As shown in the embodiment set forth in FIG. 5, the distillatetreatment plant 181 is optional, especially in the case of the use ofonce through steam generators, and in such instance the distillate 180may often be sent directly to once-through steam generators as feedwater12F′ (as distinguished from the higher quality from feedwater 12Fdiscussed hereinabove with respect to prior art processes) forgeneration of 80% quality steam 14. Also, as shown in FIG. 4, adistillate treatment plant 181 may also be optional in some cases,depending on feedwater chemistry, and in such cases, distillate 180 maybe fed directly to boiler 80 as indicated by broken line 81.

In an embodiment where boilers 80 are used rather than once throughsteam generators 12, however, it may be necessary or desirable to removethe residual organics and other residual dissolved solids from thedistillate 180 before feed of distillate 180 to the boilers 80. Forexample, as illustrated in FIG. 4, in some cases, it may be necessary toremove residual ions from the relatively pure distillate 180 produced bythe evaporator 140. In most cases the residual dissolved solids in thedistillate involve salts other than hardness. In one embodiment, removalof residual dissolved solids can be accomplished by passing theevaporator distillate 180, after heat exchanger 200, through an ionexchange system 202. Such ion-exchange systems may be of mixed bed typeor include an organic trap, and directed to remove the salts and/ororganics of concern in a particular water being treated. In any event,regenerant chemicals 204 will ultimately be required, which regenerationresults in a regeneration waste 206 that must be further treated.Fortunately, in the process scheme described herein, the regenerationwaste 206 can be sent back to the evaporator feed tank 210 for a furthercycle of treatment through the evaporator 140.

In another embodiment, removal of residual dissolved solids can beaccomplished by passing the evaporator distillate 180 through a heatexchanger 200′ and then through electrodeionization (EDI) system 220.The EDI reject 222 is also capable of being recycled to evaporator feedtank 210 for a further cycle of treatment through the evaporator 140.

The just described novel combination of process treatment steps producesfeedwater of sufficient quality, and in economic quantity, for use inpackaged boilers 80 in heavy oil recovery operations. Advantageously,when provided as depicted in FIG. 4 a single liquid waste stream isgenerated, namely evaporator blowdown 230, which contains theconcentrated solutes originally present in feedwater 46, along withadditional contaminants from chemical additives (such as regenerationchemicals 204). Also, in many cases, even the evaporator blowdown 230can be disposed in an environmentally acceptable manner, which,depending upon locale, might involve injection in deep wells 240.Alternately, evaporation to complete dryness in a zero discharge system242, such as a crystallizer or drum dryer, to produce dry solids 244 fordisposal, may be advantageous in certain locales.

Various embodiments for new process method(s), as set forth in FIGS. 4and 5 for example, are useful in heavy oil production since theygenerally offer one or more of the following advantages: (1) eliminatemany physical-chemical treatment steps commonly utilized previously inhanding produced water (for example, lime softening, filtrating, ionexchange systems, and certain de-oiling steps are eliminated); (2)result in lower capital equipment costs, since the evaporative approachto produced water treatment results in a zero liquid discharge systemfootprint size that is about 80% smaller than that required if a priorart physical-chemical treatment scheme is utilized, as well aseliminating vapor/liquid separators and reducing the size of the boilerfeed system by roughly 20%; (3) result in lower operating costs forsteam generation; (4) eliminate the production of softener sludge, thuseliminating the need for the disposal of the same; (5) eliminate otherwaste streams, thus minimizing the number of waste streams requiringdisposal; (6) minimize the materiel and labor required for maintenance;(7) reduce the size of water de-oiling equipment in most operations; and(8) decouple the de-oiling operations from the steam generationoperations.

One of the significant economic advantages of using a vertical tube,falling film evaporator such as of the type described herein is that theon-line reliability and redundancy available when multiple evaporatorsare utilized in the treatment of produced water. An evaporative basedproduced water treatment system can result in an increase of from about2% to about 3% or more in overall heavy oil recovery plant availability,as compared to a produced water treatment system utilizing aconventional prior art lime and clarifier treatment process approach.Such an increase in on-line availability relates directly to increasedoil production and thus provides a large economic advantage over thelife of the heavy oil recovery plant.

In the process disclosed herein, the evaporator 140 is designed toproduce high quality distillate (typically 2-5 ppm non-volatile TDS)which, after temperature adjustment to acceptable levels in heatexchangers 200 or 200′ (typically by cooling to about 45° C., or lower)can be fed directly into polishing equipment (EDI system 220, ionexchange system 202, or reverse osmosis system 224) for final removal ofdissolved solids. The reject stream 221 from the reverse osmosis systemcan be recycled to the evaporator feed tank 210 for further treatment.Likewise, the reject from the EDI system may be recycled to theevaporator feed tank 210 for further treatment. Similarly, theregenerant from most ion exchange processes 202 may be recycled to theevaporator feed tank 210 for further treatment. The water productproduced by the polish equipment just mentioned is most advantageouslyused as feedwater for the packaged boiler 80. That is because in thetypical once-though steam generator 12 used in oil field operations, itis normally unnecessary to incur the additional expense of finalpolishing by removal of residual total dissolved solids from theevaporator distillate stream 180. In some applications, final polishingis not necessary when using conventional boilers 80. This can be furtherunderstood by reference to FIG. 6, where a typical boiler feed waterchemistry specification is presented for (a) packaged boilers, and (b)once-through steam generators. It may be appropriate in some embodimentsfrom a heat balance standpoint that the de-oiled produced waters 46 fedto the evaporator for treatment be heated by heat exchange with thedistillate stream 180. However, if the distillate stream is sentdirectly to once-through steam generators 12, then no cooling of thedistillate stream 180 may be appropriate. Also, in the case ofonce-through steam generators 12, it may be necessary or appropriate toutilize a plurality of flash tanks F1, etc., in the manner describedabove with reference to FIG. 2.

Also, as briefly noted above, but significantly bears repeating, inthose cases where the EDI system 220 is utilized for polishing, themembrane reject stream includes an EDI reject stream 222 that isrecycled to be mixed with the de-oiled produced water 46 in theevaporator feed tank 210 system, for reprocessing through the evaporator140. Similarly, when reverse osmosis is utilized the a membrane rejectstream includes the RO reject stream which is recycled to be mixed withthe de-oiled produced water 46 in the evaporator feed tank 210 system,for reprocessing through the evaporator 140. Likewise, when ion-exchangesystem 202 is utilized, the regenerant waste stream 206 is recycled tobe mixed with the de-oiled produced water 46 in the evaporator feed tanksystem, for reprocessing through the evaporator 140.

Again, it should be emphasized that the blowdown 230 from the evaporator140 is often suitable for disposal by deep well 240 injection.Alternately, the blowdown stream can be further concentrated and/orcrystallized using a crystallizing evaporator, or a crystallizer, inorder to provide a zero liquid discharge 242 type operation. This is animportant advantage, since zero liquid discharge operations may berequired if the geological formation is too tight to allow waterdisposal by deep well injection, or if regulatory requirements do notpermit deep well injection.

Many produced waters encountered in heavy oil production are high insilica, with values that may range up to about 200 mg/l as SiO₂, orhigher. Use of a seeded slurry operational configuration in evaporator140 co-precipitates silica with precipitating calcium sulfate, toprovide a process design which prevents the scaling of the innersurfaces 260 of the heat transfer tubes 156 with the ever-presentsilica. This is important, since silica solubility must be accounted forin the design and operation of the evaporator 140, in order to preventsilica scaling of the heat transfer surfaces 260.

Since the calcium hardness and sulfate concentrations of many producedwaters is low (typically 20-50 ppm Ca as CaCO3), it is possible in manycases to operate the evaporators 140 with economically efficientconcentration factors, while remaining below the solubility limit ofcalcium sulfate, assuming proper attention to feedwater quality and topre-treatment processes.

It is to be appreciated that the water treatment process describedherein for preparing boiler feedwater in heavy oil recovery operationsis an appreciable improvement in the state of the art of water treatmentfor oil recovery operations. The process eliminates numerous of theheretofore encountered waste streams, while processing water in reliablemechanical evaporators, and in one embodiment, in mechanical vaporrecompression (“MVR”) evaporators. Polishing, if necessary, can beaccomplished in ion exchange, electrodeionization, or reverse osmosisequipment. The process thus improves on currently used treatment methodsby eliminating most treatment or regeneration chemicals, eliminatingmany waste streams, eliminating some types of equipment. Thus, thecomplexity associated with a high number of treatment steps involvingdifferent unit operations is avoided.

In the improved water treatment method, the control over waste streamsis focused on a the evaporator blowdown, which can be convenientlytreated by deep well 240 injection, or in a zero discharge system 242such as a crystallizer and/or spray dryer, to reduce all remainingliquids to dryness and producing a dry solid 244. This contrasts sharplywith the prior art processes, in which sludge from a lime softener isgenerated, and in which waste solids are gathered at a filter unit, andin which liquid wastes are generated at an ion exchange system and inthe steam generators. Moreover, this waste water treatment process alsoreduces the chemical handling requirements associated with watertreatment operations.

It should also be noted that the process described herein can beutilized with once through steam generators, since due to the relativelyhigh quality feedwater—treated produced water—provided to such oncethrough steam generators, the overall blowdown rate of as low as about5% or less may be achievable in the once through steam generator.Alternately, as shown in FIG. 5, at least a portion of the liquidblowdown 134 from the once through steam generator 12 can be recycled tothe steam generator 12, such as indicated by broken line 135 to feedstream 12F′.

In yet another embodiment, to further save capital and operatingexpense, industrial boilers of conventional design may be utilized sincethe distillate—treated produced water—may be of sufficiently goodquality to be an acceptable feedwater to the boiler, even if it requiressome polishing. It is important to observe that use of such boilersreduces the boiler feed system and evaporative produced water treatmentsystem size by twenty percent (20%), eliminates vapor/liquid separationequipment as noted above, and reduces the boiler blowdown flow rate byabout ninety percent (90%).

In short, evaporative treatment of produced waters using a falling film,vertical tube evaporator is technically and economically superior toprior art water treatment processes for heavy oil production. It ispossible to recover ninety five percent (95%) or more, and even up toninety eight percent (98%) or more, of the produced water as highquality distillate 180 for use as high quality boiler feedwater(resulting in only a 2% boiler blowdown stream which can be recycled tothe feed for evaporator 140). Such a high quality distillate stream maybe utilized in SAGD and non-SAGD heavy oil recovery operations. Such ahigh quality distillate stream may have less than 10 mg/L ofnon-volatile inorganic TDS and is useful for feed either to OTSGs or toconventional boilers.

The overall life cycle costs for the novel treatment process describedherein are significantly less than for a traditional lime softening andion exchange treatment system approach. And, an increase of about 2% to3% in overall heavy oil recovery plant availability is achievedutilizing the treatment process described herein, which directly resultsin increased oil production from the facility. Since boiler blowdown issignificantly reduced, by as much as 90% or more, the boiler feed systemmay be reduced in size by as much as fifteen percent (15%) or more.Finally, the reduced blowdown size results in a reduced crystallizersize when zero liquid discharge is achieved by treating blowdown streamsto dryness.

Although only several exemplary embodiments of this invention have beendescribed in detail, it will be readily apparent to those skilled in theart that the novel produced waste treatment process, and the apparatusfor implementing the process, may be modified from the exact embodimentsprovided herein, without materially departing from the novel teachingsand advantages provided by this invention, and may be embodied in otherspecific forms without departing from the spirit or essentialcharacteristics thereof. Therefore, the disclosures presented herein areto be considered in all respects as illustrative and not restrictive. Itwill thus be seen that the objects set forth above, including those madeapparent from the preceding description, are efficiently attained. Manyother embodiments are also feasible to attain advantageous resultsutilizing the principles disclosed herein. Therefore, it will beunderstood that the foregoing description of representative embodimentsof the invention have been presented only for purposes of illustrationand for providing an understanding of the invention, and it is notintended to be exhaustive or restrictive, or to limit the invention onlyto the precise forms disclosed.

All of the features disclosed in this specification (including anyaccompanying claims, and the drawing) may be combined in anycombination, except combinations where at least some of the features aremutually exclusive. Alternative features serving the same or similarpurpose may replace each feature disclosed in this specification(including any accompanying claims, and the drawing), unless expresslystated otherwise. Thus, each feature disclosed is only one example of ageneric series of equivalent or similar features. Further, while certainprocess steps are described for the purpose of enabling the reader tomake and use certain water treatment processes shown, such suggestionsshall not serve in any way to limit the claims to the exact variationdisclosed, and it is to be understood that other variations, includingvarious treatment additives or alkalinity removal techniques, may beutilized in the practice of my method.

The intention is to cover all modifications, equivalents, andalternatives falling within the scope and spirit of the invention, asexpressed herein above and in any appended claims. The scope of theinvention, as described herein and as indicated by any appended claims,is thus intended to include variations from the embodiments providedwhich are nevertheless described by the broad meaning and range properlyafforded to the language of the claims, as explained by and in light ofthe terms included herein, or the legal equivalents thereof.

1. A process for treatment of a produced water stream resulting from theproduction of oil from heavy oil reserves, said produced water streamcomprising dissolved solutes including calcium, sulfate, silica,alkalinity, and non-condensable gases, and wherein said treatmentcomprises: (a) acidification of said produced water stream, to removealkalinity by conversion to free carbon dioxide; (b) steam strippingsaid produced water stream to remove said non-condensable gases and saidcarbon dioxide produced in said alkalinity removal step; (c) evaporationof said produced water stream to produce a slurry comprising water,dissolved solutes, calcium sulfate, and silica, to generate (i) adistillate stream at about 95% or more by volume of said produced waterstream and (ii) a blowdown stream at about 5% or less by volume of saidproduced water stream; (d) generating a steam stream at about 100%quality and at about 1000 pounds per square inch pressure or more fromsaid distillate stream, and wherein said steam stream comprises at leastabout 70% by weight of said distillate stream, and generating a blowdownstream of about 30% or less by weight of said distillate stream.
 2. Theprocess as set forth in claim 1, wherein said steam stream comprises atleast 95% by weight of said distillate stream.
 3. The process as setforth in claim 2, wherein said steam stream comprises at least 98% byweight of said distillate stream.
 4. The process as set forth in claim3, where said blowdown stream is added to and mixed with said producedwater stream prior to evaporation of said produced water stream.
 5. Theprocess as set forth in claim 3, wherein evaporation of said producedwater stream is accomplished in a falling film evaporator.
 6. Theprocess as set forth in claim 1, wherein the produced water stream istreated with acid to lower the pH to convert alkalinity to free carbondioxide.
 7. The process as set forth in claim 1, wherein said evaporatorcomprises a heat transfer surface, and wherein said slurry of calciumsulfate and silica is maintained at a preselected concentration forpreferential precipitation of said calcium said sulfate and said silicato said slurry rather than to said heat transfer surfaces of saidevaporator.